In this experiment, the transistor (2N3904 NPN Bipolar Junction Transistor) is acting as a switch. In the first picture there exists a closed circuit between the battery, 1k ohm resistor, LED, and transistor, but the LED is not lighted therefore current is not flowing. This is because the transistor is open and functioning as an electronic switch. When the on/off switch is turned on, current is applied to the base of the transistor closing the electronic switch (transistor) completes the circuit, which allows current to flow in the circuit. In the pictures below, I take voltage reading before and after turning the switch on. The first two pictures are at the collector. Notice the voltage before and after the switch is turned on or off. The next two at the positive side of the LED. The following two at the base. When the switch is turned on, 0.78 volts is needed to release the voltage between the collector and emitter. And the last two, the emitter. Note: all the readings are in volts except the last one. That is in millivolt because there is always a voltage drop across the NPN emitter.
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| Switch is OFF Voltage is collected at the Collector |
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| Switch is ON Voltage is passing to the LED |
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| Voltage at Diode with switch OFF. |
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| Voltage at Diode when switch is ON. |
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